It is well-known that wind flow over a microphone will induce significant amounts of low frequency noise. For example, signals from microphones used in wind tunnels, active-noise control systems and other flow related systems are often corrupted by flow-induced noise. Moreover, such flow-induced noise is a problem for various types of voice transmission and sound recording systems. It is a particular problem for digital speech encoding systems and communication handsets such as cellular phones, when used, for example, outdoors or by open windows in automobiles. For such devices, flow-induced noise can also be caused by the users as they speak into the device.
In digital speech encoding systems, the speech signal is not only degraded by the transmission of the flow-induced noise itself (that is, the listener on the other end would hear noise as well as speech, as would be the case in analog systems), but also by the artifacts and distortion created as the noise is passed through speech encoding-decoding processing. As the speech decoder attempts to recreate the noise as speech, this distortion makes the speech itself difficult for the listener to perceive.
Although there are several devices in the prior art that attempt to eliminate or reduce microphone flow noise, they generally are not acceptable for such applications as cellular phones. As cellular phones become increasingly compact and the parts contained therein more miniaturized, there is less and less space available to accommodate these prior art devices. For example, microphones may be enclosed within windsocks, wind shrouds or windscreens of various constructions. These devices may be ineffective unless their dimensions are on the order of many centimeters and larger. In addition, they typically use foam, which deteriorates over time. Their size requirement and lack of durability make them unacceptable for cellular phone and other similar applications.
Unfortunately, other prior art techniques are similarly inapplicable for cellular phones. For example, as noted above, the trend to produce cellular phones that are as compact as possible encourages the miniaturization of component parts. This approach disfavors the well-known technique of increasing the diameter of the microphone to reduce the effect of flow noise. Active Noise Control (ANC), which necessitates the placement of a fluid dynamics sensor at a very small distance from the microphone, cannot be applied robustly and inexpensively for cellular phones. Only fragile sensors, such as a "hot" wire, are able to be placed close enough to the microphone. Further, the extra sensor, circuitry and processing required for ANC would give rise to additional costs.
Therefore, the problem of flow-induced noise for microphones in applications such as cellular phones remains to be addressed. Ideally, this noise should be reduced without adversely affecting the desired acoustic signal and without exceeding critical space limitations.